arXiv subjects

T. Motoba

Publications and source records attributed to T. Motoba.

At least 19 recordsLinked to original sources

arXiv2018

Relativistic Mean-Field and Beyond Approaches for Deformed Hypernuclei

We report the recent progress in relativistic mean-field (RMF) and beyond approaches for the low-energy structure of deformed hypernuclei. We show that the $Λ$ hyperon with orbital angular momentum $\ell=0$ (or $\ell>1$) generally reduces (enhances) nuclear quadrupole collectivity. The beyond mean-field studies of hypernuclear low-lying states demonstrate that there is generally a large configuration mixing between the two components $[^{A-1}Z (I^+) \otimes Λp_{1/2}]^J$ and $[^{A-1}Z (I\pm2 ^+) \otimes Λp_{3/2}]^J$ in the hypernuclear $1/2^-_1, 3/2^-_1$ states. The mixing weight increases as the collective correlation of nuclear core becomes stronger. Finally, we show how the energies of hypernuclear low-lying states are sensitive to parameters in the effective $N Λ$ interaction, the uncertainty of which has a large impact on the predicted maximal mass of neutron stars.

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arXiv2018

Disappearance of nuclear deformation in hypernuclei: a perspective from a beyond-mean-field study

The previous mean-field calculation [Myaing Thi Win and K. Hagino, Phys. Rev. C{\bf 78}, 054311 (2008)] has shown that the oblate deformation in $^{28,30,32}$Si disappears when a $Λ$ particle is added to these nuclei. We here investigate this phenomenon by taking into account the effects beyond the mean-field approximation. To this end, we employ the microscopic particle-rotor model based on the covariant density functional theory. We show that the deformation of $^{30}$Si does not completely disappear, even though it is somewhat reduced, after a $Λ$ particle is added if the beyond-mean-field effect is taken into account. We also discuss the impurity effect of $Λ$ particle on the electric quadrupole transition, and show that an addition of a $Λ$ particle leads to a reduction in the $B(E2)$ value, as a consequence of the reduction in the deformation parameter.

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arXiv2017

Transition from vibrational to rotational characters in low-lying states of hypernuclei

In order to clarify the nature of hypernuclear low-lying states, we carry out a comprehensive study for the structure of $^{144-154}_{~~~~~~~~Λ}$Sm-hypernuclei, which exhibit a transition from vibrational to rotational characters as the neutron number increases. To this end, we employ a microscopic particle-core coupling scheme based on a covariant density functional theory. We find that the positive-parity ground-state band in the hypernuclei shares a similar structure to that of the corresponding core nucleus. That is, regardless of whether the core nucleus is spherical or deformed, each hypernuclear state is dominated by the single configuration of the $Λ$ particle in the $s_{1/2}$ state ($Λs_{1/2}$) coupled to one core state of the ground band. In contrast, the low-lying negative-parity states mainly consist of $Λp_{1/2}$ and $Λp_{3/2}$ configurations coupled to plural nuclear core states. We show that, while the mixing amplitude between these configurations is negligibly small in spherical and weakly-deformed nuclei, it strongly increases as the core nucleus undergoes a transition to a well-deformed shape, being consistent with the Nilsson wave functions. We demonstrate that the structure of these negative-parity states with spin $I$ can be well understood based on the $LS$ coupling scheme, with the total orbital angular momentum of $L=[I\otimes 1]$ and the spin angular momentum of $S=1/2$.

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arXiv2016

Low-energy hypernuclear spectra with microscopic particle-rotor model with relativistic point coupling hyperon-nucleon interaction

We extend the microscopic particle-rotor model for hypernuclear low-lying states by including the derivative and tensor coupling terms in the point-coupling nucleon-$Λ$ particle ($NΛ$) interaction. Taking $^{13}_{~Λ}$C as an example, we show that a good overall description for excitation spectra is achieved with four sets of effective $NΛ$ interaction. We find that the $Λ$ hyperon binding energy decreases monotonically with increasing the strengths of the high-order interaction terms. In particular, the tensor coupling term decreases the energy splitting between the first $1/2^-$ and $3/2^-$ states and increases the energy splitting between the first $3/2^+$ and $5/2^+$ states in $^{13}_{~Λ}$C.

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arXiv2015

Beyond-mean-field approach to low-lying spectra of $Λ$ hypernuclei

Taking the hypernucleus $^{13}_{~Λ}$C as an example, we illustrate the miscroscopic particle-rotor model for low-lying spectra of hypernuclei. This approach is based on the beyond-mean-field method, with the particle number and angular momentum projections. The quantum fluctuation of the mean-field is also taken into account for the core nucleus using the generator coordinate method. We show that the impurity effect of $Λ$ hyperon, such as a change in $B(E2)$, is well described with this model. Our calculation indicates that the most important impurity effect in $sd$-shell hypernuclei is a change in a deformation parameter rather than in a nuclear size.

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arXiv2015

Resonant states of neutron-rich $Λ$ hypernucleus $^7_Λ$He

The structure of neutron-rich $Λ$ hypernucleus, $^7_Λ$He is studied within the framework of an $α+Λ+n+n$ four-body cluster model. We predict second $3/2^+$ and $5/2^+$ states, corresponding to a $0s$ $Λ$ coupled to the second $2^+$ state of $^6$He, as narrow resonant states with widths $Γ\sim 1$ MeV to be at 0.03 MeV and 0.07 MeV respect to the $α+Λ+n+n$ threshold. From an estimation of the differential cross section for the $^7{\rm Li} (γ,K^+) ^7_Λ$He reaction, there is a possibility to observe these state at JLab in the future. We also calculate the second $2^+$ state of $^6$He as resonant state within the framework of an $α+n+n$ three-body cluster model. Our result is $2.81$ MeV with $Γ=$4.63 MeV with respect to the $α+n+n$ threshold. This energy position is $\sim 1$ MeV higher, and with a much broader decay width, than the recent SPIRAL data. It is suggested that an experiment at JLab to search for the second $3/2^+$ and $5/2^+$ states of $^7_Λ$He would provide an opportunity to confirm the second $2^+$ state of the core nucleus $^6$He.

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arXiv2015

Microscopic study of low-lying spectra of $Λ$ hypernuclei based on a beyond-mean-field approach with covariant energy density functional

We present a detailed formalism of the microscopic particle-rotor model for hypernuclear low-lying states based on a covariant density functional theory. In this method, the hypernuclear states are constructed by coupling a hyperon to low-lying states of the core nucleus, which are described by the generator coordinate method (GCM) with the particle number and angular momentum projections. We apply this method to study in detail the low-lying spectrum of $^{13}_{~Λ}$C and $^{21}_{~Λ}$Ne hypernuclei. We also briefly discuss the structure of $^{155}_{~~Λ}$Sm as an example of heavy deformed hypernuclei. It is shown that the low-lying excitation spectrum with positive parity states of the hypernuclei, which are dominated by $Λ$ hyperon in $s$-orbital coupled to the core states, are similar to that for the corresponding core states, while the electric quadrupole transition strength, $B(E2)$, from the 2$^+_1$ state to the ground state is reduced according to the mass number of the hypernuclei. Our study indicates that the energy splitting between the first 1/2$^-$ and 3/2$^-$ hypernuclear states is generally small for all the hypernuclei which we study. However, their configurations depend much on the properties of a core nucleus, in particular on the sign of deformation parameter. That is, the first $1/2^-$ and $3/2^-$ states in $^{13}_{~Λ}$C are dominated by a single configuration with $Λ$ particle in the $p$-wave orbits and thus providing good candidates for a study of the $Λ$ spin-orbit splitting. On the other hand, those states in the other hypernuclei exhibit a large configuration mixing and thus their energy difference cannot be interpreted as the spin-orbit splitting for the $p$-orbits.

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arXiv2014

The experiments with the High Resolution Kaon Spectrometer at JLab Hall C and the new spectroscopy of ${}^{12}_Λ\text{B}$ hypernuclei

Since the pioneering experiment, E89-009 studying hypernuclear spectroscopy using the $(e,e^{\prime}K^+)$ reaction was completed, two additional experiments, E01-011 and E05-115, were performed at Jefferson Lab. These later experiments used a modified experimental design, the "tilt method", to dramatically suppress the large electromagnetic background, and allowed for a substantial increase in luminosity. Additionally, a new kaon spectrometer, HKS (E01-011), a new electron spectrometer, HES, and a new splitting magnet were added to produce precision, high-resolution hypernuclear spectroscopy. These two experiments, E01-011 and E05-115, resulted in two new data sets, producing sub-MeV energy resolution in the spectra of ${}^{7}_Λ\text{He}$, ${}^{12}_Λ\text{B}$ and ${}^{28}_Λ\text{Al}$ and ${}^{7}_Λ\text{He}$, ${}^{10}_Λ\text{Be}$, ${}^{12}_Λ\text{B}$ and ${}^{52}_Λ\text{V}$. All three experiments obtained a ${}^{12}_Λ\text{B}$, spectrum, which is the most characteristic $p$-shell hypernucleus and is commonly used for calibration. Independent analyses of these different experiments demonstrate excellent consistency and provide the clearest level structure to date of this hypernucleus as produced by the $(e,e^{\prime}K^+)$ reaction. This paper presents details of these experiments, and the extraction and analysis of the observed ${}^{12}_Λ\text{B}$ spectrum.

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arXiv2014

Microscopic particle-rotor model for low-lying spectrum of Lambda hypernuclei

We propose a novel method for low-lying states of hypernuclei based on the particle-rotor model, in which hypernuclear states are constructed by coupling the hyperon to low-lying states of the core nucleus. In contrast to the conventional particle-rotor model, we employ a microscopic approach for the core states, that is, the generator coordinate method (GCM) with the particle number and angular momentum projections. We apply this microscopic particle-rotor model to $^9_Λ$Be employing a point-coupling version of the relativistic mean-field Lagrangian. A reasonable agreement with the experimental data for the low-spin spectrum is achieved using the $ΛN$ coupling strengths determined to reproduce the binding energy of the $Λ$ particle.

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arXiv2010

Five-body cluster structure of double-$Λ$ hypernucleus $^{11}_{ΛΛ}$Be

Energy levels of the double $Λ$ hypernucleus, $^{11}_{ΛΛ}$Be are calculated within the framework of an $ααn ΛΛ$ five-body model. Interactions between constituent particles are determined so as to reproduce reasonably the observed low-energy properties of the $αα$, $ααn$ nuclei and the existing data for $Λ$-binding energies of the $αΛ$, $ααΛ$, $αn Λ$ and $ααn Λ$ systems. An effective $ΛΛ$ interaction is constructed so as to reproduce, within the $αΛΛ$ three-body model, the $B_{ΛΛ}$ of $^6_{ΛΛ}$He, which was extracted from the emulsion experiment, the NAGARA event. With no adjustable parameters for the $ααn ΛΛ$ system, $B_{ΛΛ}$ of the ground and bound excited states of $^{11}_{ΛΛ}$Be are calculated with the Gaussian Expansion Method. The Hida event, recently observed at KEK-E373 experiment, is interpreted as an observation of the ground state of the $^{11}_{ΛΛ}$Be.

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arXiv2009

Structure of A=7 iso-triplet $Λ$ hypernuclei studied with he four-body model

The structure of the T=1 iso-triplet hypernuclei, $^7_Λ$He, $^7_Λ$Li and $^7_Λ$Be within the framework of an $α+Λ+N+N$ four-body cluster model is studied. Interactions between the constituent subunits are determined so as to reproduce reasonably well the observed low-energy properties of the $αN$, $αΛ$, $αNN$ and $αΛN$ subsystems. Furthermore, the two-body $ΛN$ interaction is adjusted so as to reproduce the $0^+$-$1^+$ splitting of $^4_Λ$H. Also a phenomenological $ΛN$ charge symmetry breaking(CSB) interaction is introduced. The $Λ$ binding energy of the ground state in $^7_Λ$He is predicted to be 5.16(5.36) MeV with(without) the CSB interaction. The calculated energy splittings of the $3/2^+$-$5/2^+$ states in $^7_Λ$He and $^7_Λ$Li are around 0.1 MeV. We point out that there is a three-layer structure of the matter distribution, $α$ particle, $Λ$ skin, proton or neutron halo, in the $^7_Λ{\rm He}(J=5/2^+)$, $^7_Λ{\rm Li}(J=5/2^+)$ and $^7_Λ{\rm Be}(J=1/2^+)$ states.

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arXiv2008

Light $Ξ$ hypernuclei in four-body cluster models

Detailed structure calculations in $^{\: 12}_{Ξ^-}$Be, $^{\: 5}_{Ξ^-}$H, $^{\: 9}_{Ξ^-}$Li, $^{\: 7}_{Ξ^-}$H and $^{\:10}_{Ξ^-}$Li are performed within the framework of the microscopic two-, three- and four-body cluster models using the Gaussian Expansion Method.

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arXiv2007

Photo- and electro-production of medium mass Lambda-hypernuclei

The characteristic and selective nature of the electro-magnetic production of Lambda-hypernuclei in exciting states is demonstrated assuming the medium-mass targets 28Si, 40Ca, and 52Cr. Formalism of DWIA is used adopting the Saclay-Lyon, Kaon-MAID, Adelseck-Saghai, and Williams-Ji-Cotanch models for the elementary amplitudes and various nuclear and hypernuclear wave functions. The elementary amplitudes are discussed in detail presenting their basic properties and comparison with data. The unique features of the electro-magnetic production of Lambda-hypernuclei shown on examples are the slective excitation of unnatural parity highest-spin states (natural parity ones for the LS-closed targets) and a possibilty to investigate the Lambda single-particle energies including a spin-orbit splitting using variety of medium-mass targets.

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arXiv2006

Four-body structure of $^7_Λ$Li and $ΛN$ spin-dependent interaction

Two spin-doublet states of %$3/2^+$-$1/2^+$ and $7/2^+$-$5/2^+$ in $^7_Λ$Li are studied on the basis of the $α+Λ+n+p$ four-body model. We employ the two-body interactions which reproduce the observed properties of any subsystems composed of $αN$, $αΛ$ and $αNN$, and $αΛN$. Furthermore, the $ΛN$ interaction is adjusted so as to reproduce the $0^+$-$1^+$ splitting of in $^4_Λ$H. The calculated energy splittings of $3/2^+$-$1/2^+$ and $7/2^+$-$5/2^+$ states in $^7_Λ$Li are 0.69 MeV and 0.46 MeV, which are in good agreement with the resent observed data. The spin-dependent components of the $ΛN$ interaction are discussed.

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arXiv2002

Four-body cluster structure of $A=7-10$ double-$Λ$ hypernuclei

Energy levels of the double-$Λ$ hypernuclei $_Λ^{}$$_Λ^7$He, $_Λ^{}$$_Λ^7$Li, $_Λ^{}$$_Λ^8$Li, $_Λ^{}$$_Λ^9$Li, $_Λ^{}$$_Λ^9$Be and $_Λ^{}$$_Λ^{10}$Be are predicted on the basis of the $α+x+Λ+Λ$ four-body model with $x=n, p, d, t, ^3$He and $α$, respectively.

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arXiv2001

$Ξ^-$ emission probabilities at $(K^-,K^+)$ reaction points and the $ΞN$ interaction

$Ξ^-$ escaping and scattering probabilities in the $(K^-,K^+)$ reaction with nuclear targets is investigated on the basis of the eikonal approximation. Calculations are performed using the $S=-2$ sector of the Nijmegen model-D interaction, whose hard core radii are treated phenomenologically. %The $Ξ^- N$, $Ξ^- p$ and $Ξ^- n$ elastic cross sections are The $Ξ^- N$ elastic cross sections are derived from the $^9$Be$(K^-,K^+)$ data of the BNL-E906 experiment, which is useful to determine the hard-core radii in $P$-states. The $Ξ$ single-particle potential energy is investigated with the G-matrix calculation in nuclear matter, which is sensitive to the hard-core radii in $S$-states. The important constraint for the $ΞN$ interaction is obtained by combining the two analyses.

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arXiv2001

`` $Λ- Σ$ conversion in $^4_Λ$He and $^4_Λ$H based on four-body calculation

Precise four-body calculations for $^4_Λ$He and $^4_Λ$H have been performed in the framework of the variational method with Jacobian-coordinate Gaussian-basis functions. All the rearrangement channels of both $NNN Λ$ and $NNN Σ$ are explicitly taken into account for the first time with the use of realistic $NN$ and $YN$ interactions. The role of $Λ$-$Σ$ conversion and the amount of the virtual $Σ$-component in $^4_Λ$He and $^4_Λ$H are discussed.

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arXiv1999

Evidence of Xi hypernuclear production in the ^{12}C(K^-,K^+)^{12}_{Xi}Be reaction

The E885 collaboration utilized the 1.8 GeV/c K^- beam line at the AGS to accumulate 3 x 10^5 (K^-,K^+) events. Xi hypernuclear states are expected to be produced through the reaction K^- + ^{12}C -> K^+ + ^{12}_{Xi}Be. The measured missing-mass spectrum indicates the existence of a signal below the threshold for free Xi production. Although the resolution was not sufficient to resolve discrete hypernuclear states, the excess of events in the region of missing mass, kinematically inaccessible in free Xi production, is compared to theoretical prediction for ^{12}_{Xi}Be production.

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